Recently, monocrystalline silicon is produced mainly by the Czhochralski process (CZ process). This process dips a seed crystal in a melt composed of molten polycrystalline silicon to allow monocrystalline silicon to grow around the seed crystal while drawing up the seed crystal gradually in accordance with the growth to produce a monocrystalline silicon rod. Usually, in order to facilitate its handling, the raw material polycrystalline silicon is cut into cylinders of about 70 cm in length, and the mass (called silicon rods) is charged in a crucible for melting and is molten therein. In this case, in order to increase the efficiency of melting, it is generally performed to suspend a silicon rod and introduce it into the remainder of a melt.
Heretofore, a silicon rod provided with a groove on the periphery thereof near its top is suspended by engaging it with a wire in the groove. As a result, the rod excluding the upper portion thereof, i.e., from a position several centimeters below the groove to the top of the rod is molten, so that the efficiency of melting is insufficient. In the case where many silicon rods are to be molten, each of the steps of attaching a wire to the rod, charging the rod into a silicon melt, and drawing up the remaining part of the rod must be repeated several times. This disadvantageously decreases the efficiency of melting operation considerably. To cope with this, a process for simultaneously suspending a plurality of silicon rods has been proposed (cf. JP-A-09-255467). However, this process is not free of the defect in that the upper portion from a position neighboring the groove in the periphery of the rod to its upper end makes a melting loss. In addition, a process is known in which silicon rods are each provided with grooves around its periphery on both upper and lower ends, respectively, two such rods are butted end-to-end, and an annular member having a U-shape cross-section is inserted into the upper and lower grooves to couple the rods to each other (cf. JP-A-08-310892). However, this process has the problem of instability since it uses a coupling member that is inserted from outside in the groove on the rod on its top to engage it therein so that the coupling member could be disengaged when it would happen to be in contact with the exterior when the coupled rods are being transported or fitted to the melting apparatus.
In the case where a silicon rod made of polycrystalline silicon is processed so as to have a conventional suspending structure in its end portion, annealing treatment intended to decrease residual stress must be practiced to the rod in order to process it without damages. When a polycrystalline silicon rod is produced from trichlorosilane and hydrogen as the raw materials by the Siemens method, a temperature difference of 100° C. or more occurs between the central part of the rod and the surface part of the rod. This causes a difference in stress between the central part and the surface part, which difference in stress remains in the rod as residual stress after completion of the reaction. Accordingly, if the rod is processed to provide therein a groove without performing annealing treatment in advance, cracks tend to occur in the rod by residual stress, thereby damaging the processed portion of the rod. On the contrary, if the annealing treatment is practiced, contamination of metals from the heating source occurs unavoidably so that the purity of polycrystalline silicon is decreased. Further, the anneal treatment needs heating at about 1,200° C., which is not only troublesome but also requires of an appliance that can afford to practice high temperature heating.